Sorry I could not locate this info but at what production number (vin #) did the DT1 tank switch to the puck mount??

I believe 13000

Found it 13000

Thanks you beat me to it

Here’s the tank change bulletin...

Dang, the bulletin says replace as a set, which includes the frame.

So yeah.... what's the hold up??

Actually the tank mounts and tank for the new designed fuel tanks and mounting was easily mounted to the frames of the 1968 DT1's that used the strap mount. Simple bolt on procedure. So now everyone understands that just because the tank has the puck mount does not mean it was not a 1968 fuel tank. As the bulletin was published in 8 Nov 1968 surely tanks were manufactured and made available prior to publishing the bulletin!

Interesting. I've never seen Yamaha (or anyone else) offer "bolt on" mounts to convert to the puck-style tank?

DEET did a tutorial years ago on modifying automotive freeze plugs to replace damaged or missing frame mounts.

This has been really interesting and solved a big issue for me.
I have an early s/n DT1 but with the tank mod and I wasn't sure of its authenticity and so was considering trying to re mod back to the stay fixing, but this article has given clarity as to why the change.
Mine has the original frame s/n matching the engine number but has the puck holders welded to the frame and the cable carrier on the rhs, under the tank, has been removed (another mystery solved) to allow the tank to slot on not drop on.
 

There is at least one service bulletin, that may be available elsewhere on this site (parts manual maybe??) that alludes to this "fix". Actually, I think it references a whole new frame. It may indicate a starting serial number. Is it possible that your bike has been 'restored' by someone previously? I could vision a scenario where the original tank was unusable, and this was a viable alternative.

Sorry, did not realize this had all been addressed before I wrote!